Note: Descriptions are shown in the official language in which they were submitted.
CA 02287140 1999-10-13
PROCESS TO FRACTL~F CONNECTING RODS AND THE LIKE WITH
RESONANCE-FATIGUE
This invention relates to a process for fracturing connecting rods and the
like.
BACKGROUND OF THE INVENTION:
(n PRIOR ART BACKGROUND
Many methods have been used in fracturing connecting rods, that include:
(i) Passing an electron beam along a desired splitting plane as in U. S.
Patent No
3,751,080.
(ii) Providing holes in the fracturing plane through which the fracturing
force is
l0 introduced as in U.S. Patent No. 3,994,054
(iii) Using heat treatment or freezing to embrittle the fracture area as in
U.S. Patent No.
4,768,694
(iv) Applying a static or an impulsive force acting perpendicular to the
fracture plane as
in U. S. Patent Nos. 4,860,419; 5,115,564; and 5,320,265.
(v) Actuating expanding mandrels using a wedge arrangement as in U.S. Patent
No
5,503,317.
However, most of the known methods for fracturing the connecting rods are
based on the same principle: application of an "outward pressure" to the crank
bore till
the generated stresses are high enough to fracture the connecting rod. Some of
these
2o methods attempted to overcome the difficulty of fracturing such high
strength material
by reducing or weakening the cracking area, by using techniques, such as, the
cryogenic
cooling and the electron beam hardening, which have a deleterious effect on
material
performance.
Since connecting rods are made of high strength materials, the fracturing
force is
required to be of big magnitude. The use of big force has a negative effect on
the quality
of the fractured connecting rod, especially, with large size connecting rods
in a high
production environment. Despite the improvements, some disadvantages still
exist such
as: plastic deformation, lack of flexibility in adapting the same technique to
difl~erent
sizes of connecting rods, repeated breakage of force exertion elements of the
machine,
3o and poor quality of the fractured connecting rod. Moreover, some techniques
are slow,
costly, and technically very elaborate.
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Before presenting the idea of the current invention, it is necessary to
discuss the
engineering principles on which the invention stands.
(II) TECHNICAL BACKGROUND
(A) Fracture Mechanics:
Strength failures of load bearing elements can be either of the yielding-
dominant
(ductile) or fracture-dominant (brittle) types. In case of a cracked element,
it may fail
due to reaching the plastic collapse or fracture condition. Collapse and
fracture are
competing conditions, and the one satisfied first will prevail.
High-strength materials are more likely to fail in fracture mode before
attaining
1o the plastic collapse strength. Since connecting rods are made of high-
strength materials,
they generally fail under tensile forces due to reaching the fracture limit
state.
Fracture may take place under one of two conditions, namely, plane stress or
plane strain, depending on the thickness of the element. In general,
connecting rods are
thick enough to sustain plane strain fracture. In the presence of a V-notch or
a crack,
fracture occurs under essentially elastic conditions with a limited plasticity
zone at the
tip of the crack.
The stress intensity factor (K), is the characterizing parameter for crack
extension. For each stress pattern, there is a corresponding value of the
stress intensity
factor. When the stress intensity factor reaches a certain value, crack
propagates and
2o collapse by fracture occurs. That critical value of the stress intensity
factor under plane
strain conditions, called the Plane Strain Fracture Toughness (Ki~), can be
considered as
a material property characterizing the crack resistance. Thus, the same value
of KI
should be obtained for a given material while testing specimens of different
geometric
shapes and sizes.
Lower temperature and faster strain rate decrease the plane strain fracture
toughness for a specific material, while increasing the length of a pre-
existing crack or
decreasing the fracturing area will increase the stress intensity factor, if
all other factors
remain unchanged.
(B) Resonance of a Structural System:
The connecting rod, with all movement and rotation constraints imposed on it
during the fracturing process, can be viewed as a structural system. Before
explaining
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CA 02287140 1999-10-13
how to achieve and make use of a resonance condition in this fracturing
technique, it is
helpful to introduce the following definitions pertaining to an idealized
structural
system with finite number of degrees of freedom:
Degrees of freedom: the number of independent displacements required to
define the displaced positions of all the masses relative to their original
positions is
called the number of degrees of freedom (DOFs).
Natural mode of vibration: a mufti-degree-of freedom system (MDOF) would
undergo simple harmonic motion, without a change of the deflected shape, if
free
vibration is initiated by appropriate distribution of displacements in various
DOFs. In
other words, for some characteristic deflected shapes, the system would
vibrate in
simple harmonic motion, and the initial shape would be maintained through out
the
motion. Each characteristic deflected shape (~") is called a natural mode of
vibration of
the MDOF system.
Natural vibration properties: the time (Tn) required for a system to complete
one
cycle of the simple harmonic motion in one of its natural modes is called the
natural
period of that particular vibration mode. The corresponding natural cyclic
frequency of
vibration is f", and the natural circular frequency of vibration is cut,,
where:
T" = 2~/c~ = 1/ f".
A vibrating system with N number of DOFs has N natural vibration frequencies
c~" (n =
1, 2,..., N), arranged in sequence from smallest to largest (wl < c~ < ... <
cue), with
corresponding natural periods Tn, and natural modes ~".
The excitation frequency: the frequency of a harmonic force applied to a
system
is called the excitation frequency or the forcing frequency.
Damping: the process by which vibration steadily diminishes in amplitude is
called damping.
SUMMARY OF THE INVENTION
The present invention employs a novel approach to fracture connecting rods. In
3o this process, several factors are used to raise the stress intensity factor
in the connecting
rod up to the fracture point. Consequently, the use of single big force has
been avoided
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CA 02287140 1999-10-13
with the application of several small magnitude forces. That eliminates many
problems
associated with the use of big forces. It also gives better control over the
fracturing
process, since the contribution of each factor is optimized to achieve the
best results.
For this process, a stress-riser should be provided in a prior process, using
any of the
known methods, in order to predetermine the fracture plane.
The present invention utilizes the following factors:
(a) Fatigue: if the stresses in a pre-notched connecting rod fluctuate due to
the
application of harmonic forces (or any time varying forces), the pre-existing
crack
(stress-riser) will extend incrementally depending on the range of fluctuation
in the
to stress intensity factor. It is important to notice that the crack growth
relates to the
change of the stress intensity factor, not to its absolute value. Moreover, as
the crack
grows, the absolute value of stress intensity factor will increase.
(b) Resonance: during the fracturing process, the connecting rod will be in
contact with many elements of the machine. These elements impose movement
constraints, called geometrical boundary conditions, to the connecting rod.
The
connecting rod, with these boundary conditions, represents a distributed mass
structural
system, with an infinite number of degrees of freedom. However, it can be
idealized and
analyzed as a system with finite number of degrees of freedoms by using the
finite
element method.
2o If a MDOF structural system is subjected to an external force system, where
the
spatial distribution of the force components is independent of time, it takes
a certain
deformed shape. This shape does not necessarily resemble any of the natural
vibration
modes of the system. However, it has the same configuration as one of these
natural
modes, and with judicious selection of the external forces, the forced
deformed shape
can present a better approximation to that mode fir, which has a natural
frequency ~. In
most cases, ~r is one of the first few natural modes.
If the force components have the same sinusoidal time variation, with a
frequency that is the same as or close to the natural frequency ~, a resonance
condition
occurs. Consequently, the fluctuation range of the stress intensity factor and
its
3o maximum value increase substantially. The crack extends, and fracture may
occur,
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CA 02287140 1999-10-13
depending on the relative magnitudes of stress intensity factor and material
fracture
toughness.
The aforementioned principle is applied in the present invention, where two
harmonic forces, with the same amplitude and a phase angle of 180°, are
applied
simultaneously to two sides of the connecting rod. The two forces act along a
straight
line parallel to the predetermined fracture plane and perpendicular to the
axis of the
bore cylindrical surface. Moreover, the clamping arrangement allows free
deformation
of major part, centered on the V-notch, of each of the two webs defining the
bore.
A suggested method, to apply the two harmonic forces, is to transform the
rotary
1o motion generated by a hydraulic motor to a couple of rotary motions, one
clockwise and
another anti-clockwise. These rotary motions can be transformed to linear
motions
using cam means, which transfer the pressure to the connecting rod through two
contacts. The use of a single motor will guarantee that there is no time lag
between the
two equal but opposite forces.
(c) Pre-stressing forces: three pre-stressing forces are applied in the
present
invention. The primary one is applied by moving an upper jaw, which is part of
the
clamping arrangement, in a direction perpendicular to the predetermined
fracture plane
and away from it. This force works to eliminate the compression stress zones
created by
the harmonic forces and to decrease the deformation due to its application,
giving more
2o rigidity to the system. Eliminating the compression zones is favorable,
since they
deviate the fracture from predetermined plane.
The secondary pre-stressing forces are two static forces, equal in magnitude
and
acting on the same straight line in opposite directions, toward the bore axis.
The same
mechanism used to apply the harmonic forces can be used to apply these forces.
Firstly,
the two contacts advance till they slightly press the part applying the
secondary pre-
stressing forces, and secondly, they move forward and backward applying the
harmonic
forces.
These forces have clamping and damping actions. However, since they act in the
same opening loading mode as the primary force, the stresses due to all of
them can be
3o superimposed. This will facilitate further reduction of the high magnitude
fracturing
forces.
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CA 02287140 1999-10-13
Under the effect of the external forces, the deformed shape of the connecting
rod
changes with time. However, during each cycle, it passes through maximum and
minimum deformation positions at time instants T,~X and To, respectively. The
stress
intensity factor corresponding to the harmonic forces has a maximum value at
the
maximum deformation position. Both of To and T,~ can be determined
analytically by
knowing the characteristics of the forces.
(d) Dynamic force: a dynamic force is finally applied at a time instant Tf, by
increasing the primary pre-stressing force suddenly as an impulsive force at
Tf, or at a
slower rate within a period centered on Tf The time instant Tf to be
determined by
1o performing several simple tests, by applying the fracturing force during
different cycles
at different time instants such as To (minimum deformation) or T~X (maximum
KI) and
comparing the quality of the fractured connecting rods.
For example, if To comes after 0.10 seconds from the beginning of each cycle,
and the natural vibration period is 0.25 second, a test can be performed by
applying an
1s impulsive force at To during the cycle 101 after 25.10 seconds from the
beginning of the
harmonic excitation. In another test, a dynamic force is applied during a
period starting
from the instant 25.05 seconds, and ending at the instant 25.15 seconds,
measured from
the beginning of the excitation. Similar tests are performed by applying the
fracturing
force during different cycles, and at different time instants, and by
comparing the
2o quality of the fractured connecting rods, Tf is identified. However, a
longer period
before applying the dynamic force, increases the fatigue effect.
All of the external forces used in the aforementioned factors are in the same
loading mode and, generally, stress the connecting rod within the linear
elastic regime.
Thus, the stress intensity factor due to their collective effect, E KI, is
obtained by adding
25 the values of Kl that correspond to each one of them, if applied
individually. Fracture
occurs when E KI = KI~.
The flexibility of the external force system used in the current invention,
makes
the technique suitable for the wide variety of connecting rods types and
sizes, starting
from those intended for light duty applications such as lawnmowers and
outboard
3o marine engines, up to the most powerful combustion engines.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the integrally formed connecting rod, where lA is the part
intended to
become the rod, 1B is the part intended to become the cap. 1 CL and 1 CR are
the stress-
riser, 1DL and 1DR are the sides where the harmonic forces and the secondary
pre-
stressing forces are applied, and lE is the predetermined fracture plane.
Figure 2 shows an idealization of a connecting rod as a structural system.
Figure 3 shows the external force system used to fracture the connecting rod,
where F1
is the primary pre-stressing force; F2L, FZR are the secondary pre-stressing
forces; F3r.,
F3R are the harmonic forces; and F4I, F4D are the impulsive and the slow rate
dynamic
1o force, respectively. It also shows the clamping arrangements and the force
exerting
elements of the machine; where 2 is the upper jaw; 3 is the lower jaw; 4L, 4R
are the two
contacts; and 6; ?; 8; 9; 10; 11 are other clamping elements.
Figure 4 is a cross sectional view taken along line 5 of Figure 3.
Figure 5 shows the deformed shape of the connecting rod under the effect of
the pre-
stressing forces and the harmonic forces. The deformed configuration is
exaggerated for
clarity.
Figure 6 shows the first three natural vibration modes of the idealized
structural system.
Notice how the first mode (Mode 1 ) has the same configuration of the deformed
shape
of the structure shown in Figure 5.
2o Figure 7 shows the different modes of loading a cracked structural element,
all forces
used in this structural system are in the opening mode.
DESCRIPTION OF THE INVENTION:
The following is a description of the two parts of the process, the stress
analysis,
and the process implementation:
(1) Stress Analysis
A stress analysis should be performed once for each type of connecting rods,
using any appropriate computer software package according to the following
steps:
(a) Creating a three dimensional solid model of the connecting rod.
(b) Idealizing the connecting rod as a structural system, using the solid
model with the
3o movements and rotation constraints that represents the geometrical boundary
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CA 02287140 1999-10-13
conditions, and determining the first few natural vibration frequencies and
mode shapes
of the system, using the finite element method.
(c) Determining the characterizing deformed shape of the system under the
effect of a
couple of harmonic forces with amplitude equal to unity, a primary pre-
stressing force,
and a couple of secondary pre-stressing forces, each with magnitude of unity,
applying
all simultaneously.
(d) Defining the natural mode shape ~~ that has the same configuration as the
deformed
shape determined in step (c), and its corresponding natural frequency cu,..
(e) Determining the deformed shapes of the system under the effect of the
forces
io described in step (c) applying each individually.
(f) Using the results of step (e), an optimum combination of the harmonic and
pre-
stressing forces is determined that produces a deformed shape closely
resembling the
modal shape fir.
(g) Determining the time instants To and T,r,aX, measured from the beginning
of any
harmonic cycle, where To is the time instant at which the deformed shape of
the
connecting rod is the closest to its original shape, TI"eX is the time instant
at which the
stress intensity factor, corresponding to the harmonic forces, has a maximum
value.
(II) Process Implementation:
Firstly, the connecting rod is clamped in position on the upper jaw 2, which
is
2o movable in direction perpendicular to the predetermined fracture plane, and
on the
lower jaw 3, which is stationary. Both jaws are in contact with the inside
cylindrical
surface of the bore, while other elements 6, 7, 8, and 9 are pressing the
connecting rod
surface against the two jaws. It is supported at points 10 and 11 as shown in
figure 3.
However, the exact locations of these points depend on connecting rod
configuration
and manufacturing requirements.
Secondly, the pre-stressing force F1 is applied, by moving the upper jaw away
from plane lE.
Thirdly, the two contacts 4 and S advances, in opposite directions, to contact
the
sides 1DL and 1DR applying the secondary pre-stressing forces F2L and FZR.
3o Fourthly, the two contacts 4L and 4R move forward and backward,
simultaneously,
applying the two harmonic forces F3L and F3R, with a phase angle of
180°, while their
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CA 02287140 1999-10-13
frequency is as close as practically possible to the selected natural
frequency of the
system.
Fifthly, an impulsive fracturing force F4I is applied at the time instant Tf,
causing
a sudden increase of Fl and, thus, fracturing the connecting rod. Tfis
determined first, by
performing the tests that were described earlier.
An alternative to execute the last step is to apply a slow rate dynamic force
within a period centered on the same time instant.
While the present description of the process incorporates all of the
aforementioned factors, eliminating the pre-stressing forces or either of them
should not
to be construed as a departure from the scope of this invention. This is a
valid option,
especially, for small size connecting rods. In this case, the procedure that
was described
earlier should be followed, with an exception that is to skip the steps
related to the
omitted force or forces.
While particular embodiments of the invention have been disclosed, it is
evident
that many alternatives and modifications will be apparent to those skilled in
the art in
light of the forgoing description. Accordingly, it is intended to cover all
such
alternatives and modifications as fall within the spirit and broad scope of
the appended
claims.
to
